Anti-lock brake systems

ABSTRACT

An anti-lock brake system for a wheeled vehicle which includes a control element for varying brake pressure and a damper piston arranged in a fluid cavity and coacting with the control element for regulating the movement of the control element during the anti-lock cycle. The dampened motion of the control element will effectuate a rapid initial and more gradual final rate of reapplication of the brake pressure.

United States Patent Sharp May 16, 1972 [54] ANTI-LOCK BRAKE SYSTEMSReferences Cited [72] Inventor: Denis Sharp, Sussex, England UNITEDSTATES PATENTS 3 Assi nee: Us 5 C oration N Y N Y 3,306,677 2/1967 Dewaret al ..303/2l 1 g p p 3,403,945 10/1968 Dewar et al.... .....303/21[22] Filed: Dec. 14, 1970 3,415,577 12/1968 Walker ..303/21 3,486,800l2/1969 Ayers, Jr.... ..303/21 [21] 3,495,882 2/1970 Stelzer .303/21 1l' Rela ed U 8 App canon Dam Primary Examiner-Edward A. Sroka [63]Continuation of Ser. No. 771,591, Oct. 29, 1968, Assistant Examiner-HaddS. Lane abandoned. Attorney-Frank R. Trifari [30] Foreign ApplicationPriority Data [57] ABSTRACT Nov. 1,1967 Great Britain ..49,705/67 Ananti-leek brake system for a wheeled vehicle which ncludes a controlelement for varying brake pressure and a [52] US. Cl ..303/2l F damperPiston arranged in a fluid Cavity and acting with the [51] Int. Cl...B60t 8/06 control element for regulating the movement of the control[58] Field of Search ..303/6 21 61 62 63 21 F element during theanti-lock cycle- The dampened MASTER CYLINDER the control element willeffectuate a rapid initial and more gradual final rate of re-applicationof the brake pressure.

PATENTEDMY 16 I972 SHEET 1 OF 2 T/ME TIME

fig.2

MASTER CYLINDER INVENTOR.

DENIS SHAR P PATENTEDHAY 16 I972 3, 663 1368 SHEET 2 OF 2 MASTERCYLINDER w PRESSURE t1 t3 TIME INVENTOR.

SH RP BY DENI$ A ANTI-LOCK BRAKE SYSTEMS This application is acontinuation of Ser. No. 771,591, filed Oct. 29, 1968, now abandoned.

This invention relates to anti-lock brake systems for wheeled vehicles,i.e. systems designed to improve braking performance by releasing awheel brake automatically if the wheel tends to lock on a slipperysurface and then permitting further braking action without the need forany change in the action of the driver. Such systems can be successfulin reducing the risk of skidding due to wheel lock and maintainingdirectional control and can also reduce braking distances.

in one known hydraulic system the fluid must be pumped continuously soas to supply circulation of oil to the brake, and the anti-lock controldevice by-passes some of the fluid from the brake cylinder when it isnecessary to relieve the pressure therein. However, the continuouslypumped supply of oil is difficult and expensive to achieve on a motorvehicle.

There are other anti-lock systems which have been designed for use withhydraulic brake apparatus of the master-cylinder type, and examples ofsuch anti-lock systems are described in British Pat. Spec. No.1,101,078.

Such systems employ a wheel speed sensor and associated actuator whichdo not form an integral part of the system and may be of known type. Theactuator is, typically electro-magnetically controlled. The sensor maybe associated with the wheel as a deceleration sensor which appliessignals to the electro-magnet via a suitable signal processing circuit.The signal processing can be done in various ways and according tovarious criteria. One simple criterion (though not necessarily the idealone) is for the sensor signals to be rendered effective or passed to theelectro-magnet only when the sensor registers a wheel deceleration whichexceeds a predetermined value. This can readily be done with anelectro-magneticsensor in the form of a generator whose output has avoltage which is a function of deceleration, and a processing circuitwhich comprises a voltage treshold detector of known type followed by asuitable amplifier. Since such circuit elements are well known in theart, more detailed examples are not considered necessary.

The signal processing can be done electrically by circuit meansinterposed between the sensor and the electro-magnetic control, and forthis reason it is unusual to regard such processing as part of theactuator, still less as part of the mechanical (e.g. hydraulic) brakecontrol means (in the usual mode of operation the processing" of thesignals is the conversion from initial sensor or wheel speed informationto a signal representing simply the required on" and off periods of theanti-lock action).

In the present invention the processed signals determine the instantswhen anti-lock (e.g. brake release) action is initiated, and to thisextent it corresponds to the conventional mode of operation. However,the invention departs from the simple mode of operation out-lined abovein that, although the instant of cessation of anti-lock action (i.e. thestart of the reapplication of the brake) may also be determined by theprocessing means, the actual mechanical reapplication of the brake isconsiderably influenced by means provided in accordance with theinvention.

The invention provides an anti-lock brake control system for a wheeledvehicle which system comprises a movable antilock control element theposition of which can be varied in order to cause brake pressurevariation, means for moving said control element to an anti-lockposition in response to locking or excessive deceleration of a wheel,means for sub sequently causing brake reapplication by moving saidelement relatively rapidly in the reverse direction to a positioncorresponding to an intermediate value of the brake pressure and thencontinuing said movement more slowly until said wheel starts again tolock or to undergo excessive deceleration, and means for causing saidintermediate value in successive cycles to tend towards an optimum value(as herein defined) for the particular road surface.

The said optimum cannot be defined exactly and it is not critical.However, it can be defined in practical terms as a brake pressure whichcan just be tolerated without causing locking of the wheel on theparticular road surface.

Preferably the change from rapid to slow movement of the control elementis caused by a damper element which is free to travel at an independentrate during each period of antilock action. These two elements can besaid to further process the basic lock-unlock" information alreadyreceived from the normal processing arrangement in such a way as tomodify the control of brake reapplication to best suit the particularexisting road conditions. For this reason said elements and associatedmeans can be referred to as a combined actuator and control" mechanism,or briefly as an actuator".

Many anti-lock control'systems employ a mechanical actuator which,should a wheel tend to lock, will reduce brake fluid pressure to a valuewhich will allow the wheel to accelerate again and is also adapted tore-apply the pressure to the initial locking value after anti-lockaction.

In a simple system the pressure is reduced to a value which will preventthe wheel locking or which will allow it to be released if it hasalready locked. The mechanical actuator can be arranged so that iteither re-applies the brake (1) quickly up to the original pressure, or(2) re-applies the brake slowly towards the original pressure. Theresultant effects will now be described for these two cases.

Case (1) The first case is satisfactory for a good surface where thebrake pressure needed is only slightly below the actual pressure appliedby the driver. Under these circumstances the wheel tends to repeatedlygo through the point of maximum adhesion.

On a poor surface, however, the brake pressure needed may beconsiderably below the actual pressure applied and, with an actuatorwhich re-applies the brake quickly, the wheel will be held for most ofthe time at, or too close to, the locked condition; this gives poordirectional control and braking which is below the maximum possible.

Case 2) if the actuator is arranged so that it applies the brakes slowlyup to the original pressure, it will leave the wheel practicallyunbraked for most of the time on a good surface. This is due to the factthat a very low pressure may be required to free the wheel (because ofhysteresis) and the subsequent slow pressure rise results in latere-braking.

On a poor surface the pressure needed to lock the wheel can be smallcompared to the pressure applied to the driver. In this case the slowapplication of the brake gives the wheel time to accelerate to the pointof maximum adhesion before full locking pressure is applied. The resultis directional control and braking which can be regarded as good whenrelated to a bad road surface.

From the above two extremes it can be deduced that an ideal system wouldbe one which will automatically adjust its performance to suit thevarious surface conditions and the applied brake pressure and a systemaccording to the present invention can provide such a performance.

When the actuator is energized it reduces the brake pressure to anunlocking level p at which the wheel accelerates again. it thenre-applies the pressure rapidly to an intermediate value p which is (orshould soon become) slightly less than the original pressure. It shouldthen increase the pressure relatively slowly to ensure that the wheelwill eventually approach lock. If in fact the intermediate (p pressureis still above the locking pressure (p then the wheel will lock againbut not so rapidly as it did the first time since the excess pressure isnot so great. This cycle is repeated until the pressure is appliedrapidly to an intermediate value just below the locking pressure,subsequently rising slowly to locking pressure; it is then reduced whenthe wheel locks again. This mode ensures maximum braking throughout thecycle and reduces the surging which is typicalof a vehicle fitted with asimple conventional system.

FIGS. 1 and 2 show brake pressure/time graphs of the above sequence ofevents for good and bad road surfaces, the excess pressure beingindicated at El, E2 etc.

' Conversely,

FIG. 3 shows diagrammatically a simple actuator and FIG. 4 shows simpleadditional means enabling it to operate in the sohisticated modeoutlined above in accordance with the invention.

FIG. shows one cycle of the brake pressure/time graph.

A typical simple actuator which is not part of the invention will now bedescribed with reference to FIG. 3.

For normal braking operation there exists a fluid path from the mastercylinder to the brake via flap valve 1. The anti-lock control element isa piston 2 which controls said valve and the brake pressure. Spring 3 issufficiently strong to prevent valve 1 from closing due to mastercylinder pressure on piston 2 until an anti-lock force 4 is applied toan auxiliary control element 5. ElementS could form part of piston 2 butis preferably separate.

when this control force is present it causes the head of element 5 tocompress spring 3 and piston 2 is forced to the right under theinfluence of the brake pressure thereby closing valve 1. Furthermovement of piston 2 which thus separates from valve 1 then displacesfluid from the brake thereby releasing the latter. The consequentremoval of force 4 allows the spring 3 to force fluid back into thebrake and open valve In order to fully understand the invention asapplied to this simple actuator a corresponding embodiment of theinvention will now be described with reference to FIG. 4.

When force 4 is applied to control element 5, piston 2 follows it and isforced to the right as before and fluid is removed from the brake.However, this system also includes a damper piston 6 in a cavity filledwith fluid. During the time that piston 2 is not restraining damperpiston 6, the latter is moved independently to the right by a spring 7at a rate determined by said spring 7 and by restriction of fluid flowcaused by orifices 8 and 9. Orifice 9 is controlled by a ring seal valveprovided (as shown) on the piston 2 and it permits a much weaker spring7 to be used than would be the case if orifice 8 only were used (thusthis two-orifice arrangement is desirable though not essential).Normally orifice 9 will be larger than orifice 8.

When force 4 is removed, the spring 3 forces the piston 2 back to theleft rapidly until the ring seal valve on the piston 2 contacts thedamper piston 6 and closes orifice 9. Thereafter the pistons 2 and 6move slowly to the left at a rate controlled by orifice 8. The instantat which piston 2 thus meets piston 6 will depend, in each cycle, onthedistance travelled (to the right) by piston 6. Since this travel isindependent, the distance travelled by piston 6 will be related to thetime available in each cycle and this in turn will be related to theduration of the application of the anti-lock force 4. This durationdeends on the quality of the road surface and on changes in the roadsurface. This is illustrated by the brake pressure graphs of FIGS. 1 and2 and the'more detailed graph of one cycle given in FIG. 5 of theaccompanying drawing. The brake pressure at which locking occurs on aparticular surface is indicated at p,, in FIG. 5 (this level varies withroad surface as will be seen by comparing the locking levels in FIGS. 1and 2). The braking action will overshoot this level p; to an extent Epreviously referred to as an excess pressure, the final pressure of theparticular cycle being indicated at p.,.

At this point the wheel looks (or tends to lock) and this causes theanti-lock means (not shown in FIGS. 3 and 4) to produce the force 4, thelatter being applied at'instant t, (FIG. 5). As a result the brakepressure drops rapidly to a value p (FIG. 5) at which the wheel isunlocked. The greater the excess pressure E, the greater the pressuredrop ga -p that must be obtained to release the brake and the longer theperiod r, during which force 4 must be maintained on element 5.

This, too, is illustrated by FIGS. 1 and 2. For a good road surface theinitial excess pressure (13,) is quite small, i.e. the wheel can takealmost the whole of the applied pressure without skidding, and theautomatic adaptive action of damper 6 is only required to reduce E, to aslightly lower value E which may be reached e.g. after only one cycle(as shown). on a slippery surface the large excess pressure E 4 of FIG.2 has to reduced considerably, and over a number of cycles, for examplefrom value I5 to a value E, as shown. This corresponds to a long periodr,r, (FIG. 5) and a long movement of the damper piston 6 whose finalposition is, in efiect, a measure of the duration t,r

In any event, the force 4 is removed at instant I: and spring 3 thenforces piston 2 rapidly to the left thereby increasing the brakepressure at a fast rate up to an intermediate pressure level p which isreached at instant t (FIGiiSJaQThis instant corresponds to the point atwhich piston 2 meets piston 6. Thereafter the motion of piston 2 isslowed down by the action of damper 6 and hence the continued rise inbrake pressure beyond occurs at a slower rate. Thus it is the varyinglength of travel of piston 2 that adjusts the level p, from cycle tocycle until said level is at an optimum valve just below the :lockinglevel p (see FIG. 2 for such a progressive adaptation ovie'r fivecycles).

Element 5 could be attached to piston 2 or form part thereof. The maindisadvantage would be that a hard application of the anti-lock force 4at an instant of low brake pressure might cause piston 2 to suck airinto the brake chamber.

The system can be applied to a pair of wheels driven by a commonpropeller shaft via a differential gear, in which case the wlieels ofthe pair can share a co r nmon valve 1 and control element 2.Alternatively, the system may be one which is applied to a single wheeland comprises a valve 1 and element 2 specific to said wheel.

If the brake fades" or the road surface improves thereby requiringfurther pressure, piston 2 will eventually move sufficiently to the leftto open the valve 1 which will then admit more fluid to the brake.

It will be seen from FIG. 1 and 2 that the eventual brake pressure afterthe automatic adaptation described above will be modulated at lowamplitude very close to the locking pressure apart from the short sharpdecreases in pressure which will normally be needed to free the wheeldue to hysteresis. Hysteresis is often particularly severe in drumbrakes due to their self-serving action.

The time constants of the damper are, to some extent,

' governed by the rate at which the force 4can be applied and removed.When the damper piston 6 is moving under the influence of spring 7 andcontrol by orifices 8 and 9 its time constant must be such that thepiston 6 has time to move a reasonable distance in the time that piston2 has moved to the right and back again. The time constant of the damperpiston 6 when it is being pushed by spring 3 with control by orifice 8only) must be longer than the above time constant but not so long thatit cannot cope with brake fade or takes too long to adapt to an improvedroad surface.

What is claimed is:

1. An anti-lock brake control system for a wheeled vehicle comprising amovable control element for varying the application of brake pressure,means for moving the control element in a first direction to reducebrake pressure in response to wheel locking, means for moving thecontrol element in a second direction to re-apply brake pressure, adamper element comprising a damper piston slidably mounted in a fluidfilled chamber for cooperation with said control element so as toregulate the rate of movement of the control element in the seconddirection so that the initial movement is at a more rapid rate than thefinal movement, and effectuating independent rates of movement of thecontrol element during brake re-application, said piston defining atleast one orifice therethrough providing a passage for the fluid andpermitting dampened motion of said piston in both directions of travel,and spring means for urging the piston toward one end of the chamber. 7

2. An anti-lock brake control system as claimed in claim 1 wherein thedamper piston has two orifices, one of which is sealed when the controlelement coacts with the damper piston during brake re-application.

3. An anti-lock brake control system as claimed in claim 2 wherein thedamper piston is arranged concentrically around the control element andextends beyond the fluid chamber and said control element is adapted formovement in a direction parallel to the direction of movement of thedamper piston.

4. An anti-lock brake control system as claimed in claim 3 furtherincluding spring means for urging the control element in the seconddirection.

5. An anti-lock brake control system as claimed in claim 4 furtherincluding a valve element operated by the control element for varyingmaster cylinder pressure to one or more brake cylinders.

6. An anti-lock brake control system for a wheeled vehicle comprising ahousing, a fluid path from a master cylinder to the vehicle brakecylinder passing through said housing, a valve movably mounted withinsaid fluid path for opening and closing said path to thereby allow andprevent fluid and pressure to be applied to said brake cylinder, apiston slidably mounted within said housing having said valve attachedat one end thereof, spring means acting on the other end of said pistonfor urging said valve to remain in the open position, control meansacting on said other end of said piston for urging said piston againstthe action of said spring to thereby allow said valve to close the fluidpath in response to the pressure from said master cylinder when saidwheel is in a locked condition so as to reduce brake fluid and brakepressure to said brake cylinder so as to release said brake, a damperelement slidably mounted on said piston in a fluid chamber forcooperative engagement and travel with said piston so as to regulate therate of movement of said piston so that the initial movement is at amore rapid rate than the final movement of said piston, said damperelement having a pair of orifices for dampening the movement thereof,one of said orifices being closed when said dampening element is incooperative engagement with said piston, and biasing means engaging saiddampening element for dampening in cooperation with one of said orificesthe travel of said dampening element in one direction, said pair oforifices dampening the movement of said dampening element in the otherdirection, whereby said valve will be cyclically opened and closedduring a wheel locking condition so that the brake pressure applied tobrake cylinder will increase and decrease until an optimum pressurevalue is reached, said damper element travelling a successivelydecreasing distance with successive cycles until said optimum pressurevalue is obtained.

* a a: i

1. An anti-lock brake control system for a wheeled vehicle comprising amovable control element for varying the application of brake pressure,means for moving the control element in a first direction to reducebrake pressure in response to wheel locking, means for moving thecontrol element in a second direction to re-apply brake pressure, adamper element comprising a damper piston slidably mounted in a fluidfilled chamber for cooperation with said control element so as toregulate the rate of movement of the control element in the seconddirection so that the initial movement is at a more rapid rate than thefinal movement, and effectuating independent rates of movement of thecontrol element during brake re-application, said piston defining atleast one orifice therethrough providing a passage for the fluid andpermitting dampened motion of said piston in both directions of travel,and spring means for urging the piston toward one end of the chamber. 2.An anti-lock brake control system as claimed in claim 1 wherein thedamper piston has two orifices, one of which is sealed when the controlelement coacts with the damper piston during brake re-application.
 3. Ananti-lock brake control system as claimed in claim 2 wherein the damperpiston is arranged concentrically around the control element and extendsbeyond the fluid chamber and said control element is adapted formovement in a direction parallel to the direction of movement of thedamper piston.
 4. An anti-lock brake control system as claimed in claim3 further including spring means for urging the control element in thesecond direction.
 5. An anti-lock brake control system as claimed inclaim 4 further including a valve element operated by the controlelement for varying master cylinder pressure to one or more brakecylinders.
 6. An anti-lock brake control system for a wheeled vehiclecomprising a housing, a fluid path from a master cylinder to the vehiclebrake cylinder passing through said housing, a valve movably mountedwithin said fluid path for opening and closing said path to therebyallow and prevent fluid and pressure to be applied to said brakecylinder, a piston slidably mounted within said housing having saidvalve attached at one end thereof, spring means acting on the other endof said piston for urging said valve to remain in the open position,control means acting on said other end of said piston for urging saidpiston against the action of said spring to thereby allow said valve toclose the fluid path in response to the pressure from said mastercylinder when said wheel is in a locked condition so as to reduce brakefluid and brake pressure to said brake cylinder so as to release saidbrake, a damper element slidably mounted on said piston in a fluidchamber for cooperative engagement and travel with said piston so as toregulate the rate of movement of said piston so that the initialmovement is at a more rapid rate than the final movement of said piston,said damper element havIng a pair of orifices for dampening the movementthereof, one of said orifices being closed when said dampening elementis in cooperative engagement with said piston, and biasing meansengaging said dampening element for dampening in cooperation with one ofsaid orifices the travel of said dampening element in one direction,said pair of orifices dampening the movement of said dampening elementin the other direction, whereby said valve will be cyclically opened andclosed during a wheel locking condition so that the brake pressureapplied to brake cylinder will increase and decrease until an optimumpressure value is reached, said damper element travelling a successivelydecreasing distance with successive cycles until said optimum pressurevalue is obtained.